1. Field of the Invention
This invention relates to a displacement detector, and more particularly to a detector which magnetically detects linear or angular displacement in a non-contact fashion and converts the detected displacement into an electric signal. The detector of the invention has a high linearity between its input and output, and one can use it as an excellent control element such as a feedback control element for a robot, a recorder pen and the like.
2. Description of the Prior Art
West German Patent No. 2511683 discloses a typical detector for magnetically detecting a displacement as illustrated in a plan view of FIG. 4A and a side view of FIG. 4B. A core 1 of U-shaped cross section is made of magnetic material with a high permeability, and a magnetizing coil 2 is wound thereon. An alternating-current (AC) power source 16 is connected to magnetizing terminals D.sub.1 and D.sub.2 of the magnetizing coil 2 for excitation of the core 1. The core 1 has a gap 20 through which a printed circuit board 3 extends, and detecting coil 4a formed on the board 3 crosses the gap 20 so as to cause linkage between the magnetic flux of the gap 20 and the detecting coil 4a. The detecting coil 4a has a number of turns each of which has an edge-like slant portion 4b, so that the linkage between the detecting coil 4a and the magnetic flux of the gap 20 varies depending on the position of the core 1 relative to the printed circuit board 3.
The exciting coil 2 can be wound either on a connecting portion of two legs of the U-shaped core 1 as shown in FIGS. 4B and 4C or on one of the two legs as shown in FIG. 4D. With such U-shaped core 1, reluctance of the gap 20 as seen from the exciting coil 2 is not uniform, and when the core 1 is excited by an AC current from the AC power source 16 the flux density in the gap 20 is not uniform. For instance, if the flux densities in the gap 20 at its closed root portion, at its central portion and at its open end portion are represented by B.sub.1, B.sub.2 and B.sub.3, respectively,. their magnitudes decreases in that order, i.e., B.sub.1 &gt;B.sub.2 &gt;B.sub.3. When the core 1 is made of high permeability material, such as ferrite, the ratio of permeability between ferrite and air is in the order of 100:1 and the above non-uniformity of the magnetic flux density in the air gap 20 is not negligible.
The U-shaped core 1 of the prior art has further shortcomings; namely, that it is difficult to mount a winding or coil on such core and a special winding machine is necessary, that the formation of the exciting coil 2 on the core 1 is comparatively difficult, that the gap 20 tends to be made wide for facilitating the winding of the exciting coil 2 and such tendency results in a reduction of the voltage induced in the detecting coil 4a, and that when a foreign magnetic substance is inadvertently brought to the proximity of the open end of the gap 20, the flux density in the gap 20 is disturbed and an error may be caused thereby.
Depending on the AC flux linkage, the detecting coil 4a produces an electric signal e across its output terminals T.sub.1 and T.sub.2. Since each turn of the detecting coil 4a has the slant portion 4b of edge shape, the above AC flux linkage of the detecting coil 4a depends on relative displacement between the core 1 and the printed circuit board 3. Hence the output signal e represents such relative displacement. More particularly, the mechanical relative displacement X between the core 1 and the board 3 as shown in FIG. 4A is converted into the electric signal e.
In FIG. 4A, if the origin of the coordinate for the displacement X is set at the extreme left end of the detecting coil 4a, and if the slant portions 4b of the adjacent turns of the detecting coil 4a are spaced in succession as shown, the relationship between the displacement X and the output e can be plotted, as shown in the graph of FIG. 5. The symbol 1' of FIG. 4A shows the core 1 at the origin of the coordinate, and the numeral 5 represents an insulating sheet of the printed circuit board 3. The solid straight line OP of FIG. 5 represents linear relation which can be achieved only when the magnetic flux density in the gap 20 of the core 1 is uniform. The vertical height of the solid line OP is proportional to the physical area of interlinkage between the detecting coil 4a and the core 1, and with the edge-shaped slant portions 4b of the detecting coil 4a, the above physical area of interlinkage varies linearly with the relative displacement X. Thus, if the magnetic flux density in the gap 20 of the core 1 is uniform, the linear relationship of the solid line OP can be achieved.
The actual magnetic flux density in the gap 20 is, however, not uniform as pointed out above, and the actual output signal e becomes, for instance, a series of curves 6, 7 and 8 of FIG. 5 which deviate from the solid line OP.
To avoid such non-linearity, a number of methods have been proposed. The gap 20 may be tapered so that it becomes narrower and its reluctance becomes smaller as it extends from the closed end toward the open end, so as to produce a substantially uniform distribution of the magnetic flux density therein. It is, however, difficult to cut the core 1 with the above-mentioned taper, and the tapered gap 20 has not been used.
Japanese Utility Model No. 1634377 (Japanese Utility Model Publication No. 33369/85) discloses a more practical method, which uses a U-shaped core 1 with two long legs and two printed circuit boards 3 disposed in the gap 20 between the two long legs. A first one of the two boards 3 has a detecting core 4a similar to that of FIG. 4A. A second one of the two boards 3 is positioned on one side of the first board 3 in an upside-down fashion and carries a detecting coil 4a' (not shown). The detecting coils 4a and 4a' have an identical shape but they are disposed in the gap 20 in a reversed fashion, and deviation from linear characteristics in the second coil 4a' is in an opposite direction to that in the first detecting coil 4a. The voltage e of the first detecting coil 4a and the voltage e' of the second detecting coil 4a' are added so as to cancel the deviations from the linear characteristics in the two voltages, and the sum (e+e') is used as the output of the displacement detector. Instead of laying the two printed circuit boards 3 side by side, the two boards 3 may be overlaid, one over the other, while disposing the inclinations of the coils 4a, 4a' in reverse directions.
Further, Japanese Utility Model No. 1695217 (Japanese Utility Model Publication No. 3684/87) teaches another solution, which uses a printed circuit board 3 carrying a first and second detecting coils 4a, 4a' (not shown) formed on opposite surfaces thereof respectively. The shape of the first detecting coil 4a is such that its magnetic flux linkage increases with the displacement X, while the second detecting coil 4a' is so shaped that its magnetic flux linkage decreases with the displacement X. Deviation from the linearity is eliminated by taking the sum of the two voltages induced in the first and second detecting coils 4a, 4a'.
The solutions of non-linearity by the above-mentioned utility models, however, have a shortcoming in that formation of the required two detecting coils 4a and 4a' tends to complicate its manufacturing process. In particular, when opposite surfaces of a printed circuit board are occupied by the two detecting coils 4a and 4a', it becomes difficult or costly to provide other means on the printed circuit board for additional functions.